Posts Tagged server

This is a linux command line reference for common operations.
Examples marked with • are valid/safe to paste without modification into a terminal, so
you may want to keep a terminal window open while reading this so you can cut & paste.
All these commands have been tested both on Fedora and Ubuntu.

mine says 3.3%wa – this is the wait time trying to write to disk. Now from there you can install the package (under most distros) called ‘sysstat’

sysstat – sar, iostat and mpstat – system performance tools for Linux

This contains several tools for trying to track down whats using the disk to write lots.

iotop – simple top-like I/O monitor. This is installed and can show you realtime whats writing to disk at any time and using what load
iostat – Report Central Processing Unit (CPU) statistics and input/output statistics for devices, partitions and network filesystems (NFS).
sar – Collect, report, or save system activity information.

If there’s plenty of cache/buffers, and sar -W 1 0 shows lots of zeroes (and possibly the occasional blip) then the disk is getting thrashed, but it’s not swap.

Running iostat -dx 1 will show you all the partitions and how hard they’re working (look at %util). If %util is consistently at or around 100 for any partition of disk, you can definitively say that the disks are getting thrashed.

If the disk has high %util, but the actual throughput (rsec/s and wsec/s) is pretty low, then it’s possible you’ve got a hardware fault or RAID rebuild going on. A hardware error might show up on a smartctl run (smartctl -a /dev/sda or whatever), looking at things like the reallocated sector count, but SMART isn’t real, well, smart, so don’t trust it too much. A RAID rebuild should show up in your RAID management (you are monitoring your hardware RAID setup, aren’t you?). A software RAID rebuild will be shown in /proc/mdstat. (cat /proc/mdstat )

For those who do not know what either are, CPanel and Virtualmin are very similar to Plesk. They are a way of managing all your virtual hosting needs if you have a server. You log into a web interface, add a domain and it automagicly reconfigures your server to accept email DNS and websites for it. You can then add email addresses easily and let users manage their own stuff. They make life very easy for those less knowledgeable about Linux and servers in general. Virtualmin is often migrated to as you can download the GPL version entirely free, and it works great. Alternatively you can pay and get a slightly better more featured version.